In this scenario,where 2 ISP is landing on a single router , we can only do load sharing. In a multihomed environment, load-balancing is not an option since BGP selects only a single best path to a destination among the BGP routes learned from the different ASes. The idea is to set a better metric for the routes in the range 1.0.0.0 to 128.0.0.0 learned from ISP_A and a better metric for the rest of the routes learned from ISP_B.
So, if you do traceroute u will find that ,
any network lower than 128.0.0.0 will exit the router via 160.20.20.2(just example of IP address assigned by uor 1st ISP), the next hop out the serial 0 interface, and the rest will exit via 150.10.10.2(IP address assigned by 2nd Backbone provider), the next hop out the serial 1 interface
In case of 2 service provider landing on different Router /on single router also it is possible :
In a multihomed environment with two ISPs, load-balancing is not a possible since BGP selects only the single best path to a destination among the BGP paths learned from different ASes. The best we can achieve in such multihomed BGP networks is load sharing. Based on predetermined policies, traffic flow is controlled using different BGP attributes.
Only with the help of your service provider you can design a network where u can use one backbone for outbound traffic and one for inbound traffic and in case of failure of one backbone the other Backbone will take the whole load on it.
EXAMPLE:-
For simplicity we assume AS 100's (UORS AS NO.)BGP routing policy is as follows:
AS100( that is uor side AS number) accepts the local routes from both providers(2 Providers are having their AS 300 & AS400) along with a default for the rest of the Internet routes.
The outbound traffic policy is as follows:
Traffic destined to AS 300 goes via the R1 - ISP(A) link." R1- Router 1 or can be possible serial 1of Router "
Traffic destined to AS 400 goes via the R2 - ISP(B) link." R2 -Router 2 or can be on the 2nd serial interface"
All other traffic should prefer default route 0.0.0.0 via the R1 - ISP(A) link.
If the R1 - ISP(A) link fails, all traffic should go via the R2 - ISP(B) link.
The inbound traffic policy is as follows:
Traffic destined for network 10.10.10.0/24 from the Internet should come from the ISP(A) - R1 link.
Traffic destined for network 10.10.20.0/24 from the Internet should come from the ISP(B) - R2 link.
If one ISP fails, the other ISP should route traffic for all networks back to AS 100 from the Internet.
Hope this will help you somewhat .
Signing out for now,
Mani
